Imagine, a few decades after the big changes, say, in the 2090s, when ordinary people have access to nanotech (the way today every fourth or fifth homeless guy wears a Walkman with sound quality that would have blasted a 1950s “Hi-Fi” enthusiast right out of his rumpus room). Suppose you could carry in a toothpaste tube the nanotech stuff to build a pretty decent one or two room house out of whatever junk happened to be lying around. And suppose that, after you were finished with it, the stuff went back into the toothpaste tube of its own accord so that you could use it again. Press, squeeze, and you’re a little less homeless—at least for the night. As ever, though, I imagine the police will still come by early in the morning with toothpaste tubes of their own, full of foam specially programmed to disassemble the hastily constructed shelters back into junk; and the again-homeless will be told to move on.